The present invention relates to a process for the preparation of a water-absorbent, foam-type polymer structure, to a water-absorbent, foam-type polymer structure obtainable by that process, to a water-absorbent, foam-type polymer structure, to a composite comprising a water-absorbent, foam-type polymer structure and a substrate, to a process for the production of the composite, to composites obtainable by that process, to the use of the water-absorbent, foam-type polymer structure, and to chemical products.
Water-absorbent polymers are referred to as superabsorbers or superabsorbent polymers, because they are capable of absorbing a multiple of their own weight in aqueous liquids with the formation of hydrogels. In practice, such superabsorbers are used, for example, in diapers to absorb urine. They have the property of retaining the absorbed liquid even under mechanical load.
Superabsorbers are nowadays used predominantly in powder form. Within the scope of simplifications to processes, it is desirable to use superabsorbers in a fixed form, for example to incorporate them into a foamed matrix. According to the prior art there are two different types of foamed systems.
The first type is a foamed matrix that contains prefabricated superabsorber granules. For example, EP-A-0 427 219 describes mixtures of superabsorbent polymers and latex foams. The foamed matrix in this case serves to fix the superabsorber granules and to distribute the liquid. However, the matrix itself makes only a very limited contribution to the absorption of the liquid.
The second type are foams which themselves consist of superabsorbent material. Three different processes are described in the literature for producing such foams. They can be produced (i) by polymerization of a monomer solution in a water-in-oil polymer dispersion, (ii) by foaming of a monomer solution and subsequent polymerization or (iii) by thermal foaming of a polymer with the addition of blowing or expanding agents.
WO 96/21680 and WO 96/21181, for example, describe a foam produced by a process of type (i). In that process, a monomer mixture that contains an emulsified aqueous phase is polymerized. The water serves to keep space available for the pores of the foam, which form later. However, the foams described in that document exhibit poor retention.
WO 94/22502 describes a foam, which is obtained by a process of type (ii) and is based on partially neutralised polyacrylates. The foam is produced by foaming a monomer mixture using a water-soluble expanding agent, such as Freon 1,1,2. The disadvantage of the foams described in that document is their poor rate of absorption.
In WO 97/17397 absorbent foams are obtained by foaming a monomer solution using blowing agents according to a process of type (ii). The disadvantage of the foams described in that document is their hardness and brittleness. It is necessary to incorporate plasticizers into the foams in order to process them.
U.S. Pat. No. 4,394,930 describes absorbent polymer foams produced by a process of type (iii). In the process described in that document, prefabricated superabsorbers, such as, for example, starch copolymers, are foamed thermally on a substrate, such as, for example, a cellulose nonwoven or a polyethylene film, with the addition of blowing agents. The foams obtained by that process are hard and have little flexibility.
WO 88/0981 also describes absorbent polymer foams obtained by a process of type (iii). A terpolymer of ethyl acrylate, Na acrylate and Na methacrylate is foamed thermally in the presence of expanding agents, such as, for example, sodium hydrogen carbonate. However, because of the low polyelectrolyte content, the foams described in that document have poor retention.
The object according to the present invention is, generally, to overcome the disadvantages arising from the prior art.
A further object underlying this present invention is to provide absorbent polymer foams, which exhibit satisfactory properties, both in respect of retention and in respect of the rate of absorption and absorption under load.
A further object according to the present invention is to provide absorbent foams, which possess a high degree of softness and flexibility, even though they consist of a homogeneous polymer, and which do not require the incorporation of prefabricated superabsorber granules.
A further object underlying this present invention is to provide a process with which such foams can be produced using starting compounds that are as inexpensive as possible and, where possible, without using halogenated hydrocarbons as expanding agents.
Finally, an object according to the present invention consists in providing composites, especially hygiene articles and components thereof, which, in addition to the absorption properties, are very comfortable to wear and, in particular, do not restrict the movements of the user.